It is conventional on an aircraft, and more particularly on a helicopter, to find actuators that are arranged in parallel or in series with flight control linkages. When arranged in parallel, such actuators are commonly referred to as “trim actuators” by the person skilled in the art. Thus, a helicopter may have a trim actuator associated with its longitudinal flight controls, a trim actuator associated with its lateral flight controls, a trim actuator associated with its collective pitch flight control, and a trim actuator associated with its yaw flight controls. Each trim actuator then performs both first and second functions.
The first function improves pilot comfort by enabling the pilot to anchor a given control in a given position. For example, by blocking the collective pitch trim actuator, the pilot no longer needs to hold the collective pitch with the appropriate lever, and can therefore pay attention to other tasks.
The second function of a trim actuator consists in enabling the neutral position of a flight control to be adjusted. Furthermore, if the aircraft is fitted with an autopilot system, the trim actuator can provide information to the autopilot system. A sensor for measuring the position of a mechanical element of a trim actuator can transmit said information to the autopilot system, which can deduce therefrom the position of the associated flight control.
Under such conditions, a trim actuator generally includes a motor for driving an outlet shaft in rotation, which shaft is connected to the associated flight control by a connecting rod. For example, when the motor is actuated by the pilot, the outlet shaft of the trim actuator rotates and moves the flight control. In contrast, when the pilot acts on the flight control, it is the flight control that causes the outlet shaft to rotate and consequently rotates the rotor of the trim actuator motor relative to its stator.
That type of actuator thus satisfies requirements. Nevertheless, it is found that in the event of the motor that forms part of the trim actuator becoming jammed, that inevitably leads to a situation that is catastrophic since the flight control becomes blocked.
It is therefore essential to be able to break the connection between the trim actuator and the associated flight control, should that be necessary. For this purpose, actuator manufacturers provide a breakable pin to act as a “fuse”, e.g. between the outlet shaft and the connecting rod connecting it to a flight control. In the event of the actuator jamming, the pilot can act on the flight control and shear the pin. Although satisfactory, that solution requires the pin to be suitably dimensioned so that the force at which it breaks is neither too small nor too great. Furthermore, the pin does not necessarily break in an optimum manner. Consequently, it is possible that the flight controls continue to be impeded by a faulty trim actuator. The presently-existing solution is thus not completely satisfactory.
As a remedy, it might be envisaged to implement a suitable torque limiter in the trim actuator, the torque limiter decoupling the trim actuator motor from the flight control above a determined level of torque. In general, torque limiters comprise first and second plates that are connected together by drive means, e.g. balls. Each of the first and second plates then has a discontinuous housing provided with a plurality of orifices that are distributed equidistantly around a circle.
Below a predetermined torque, each ball is held in place, being inserted firstly in an orifice in the first plate and secondly in an orifice in the second plate. The first plate can thus drive the second plate in rotation via a connection via an obstacle, and vice versa. Above the predetermined torque, each ball leaves its orifice, thereby enabling the torque to be limited at said predetermined value, and subsequently penetrates into the next orifice. Since the balls continuously leave and enter the orifices, it will be understood that this technology is normally not suitable for being transposed into a trim actuator. That would require the pilot to fight continuously in order to work the flight controls in the event of the actuator motor jamming.
Furthermore, document U.S. Pat. No. 2,401,992 describes a device for coupling together first and second main shafts, the device being provided with blocking means, compression means, and drive means. The blocking means comprises a conical inside face provided with a first housing that is discontinuous and a second housing that is continuous.
Above a limit torque, the balls leave the first housing, slide over the conical face of the blocking means, and drop into the second housing. In order to reset that device, it then suffices to implement a simple axial action, since the device is reversible. Under such circumstances, that operation does not guarantee the operator to inspect the device as a safety precaution.